The invention relates to a method for the direct phase-angle measurement of radiation in accordance with light radiation reflected by a body (3) or passing through a transparent body, in which the body (3) is exposed to coherent radiation (2) of pre-determined frequency or the body (3) is coated with a lacquer in which particle diffusely reflecting the radiation are stored and which is exposed to non-coherent radiation (2) of a pre-determined frequency, the radiation reflected by the body (3) or the radiation which has passed through the body being imaged by an imaging optical system (6) in an image plane (7) in which a sensor (8) is located, a reference radiation generated in accordance with a shearing method being superimposed on the sensor (8), and the phase of the radiation (5) from the body (3) being determined from the measurement signals of the sensor (8). It further relates to an apparatus for the performance of such a method.
A method for the direct phase-angle measurement of radiation, in particular of light radiation, and an apparatus for the direct phase-angle measurement of radiation, in particular of light radiation, are known from EP 0 419 936 B1. In the prior known methods, a body is exposed to coherent radiation of a pre-determined frequency. The body can possess a diffuisely reflecting surface. However, it is also possible for it to be a transparent or partially transparent body or a transparent medium through which the radiation passes. Furthermore, the body can have or be provided with a lacquer coating in which particles diffusely reflecting the radiation are incorporated; in this case, a non-coherent radiation of a pre-determined frequency is sufficient to perform the method. The radiation reflected from the body or the radiation which has passed through the body or the transparent medium is imaged by an imaging optical system in an image plane in which a sensor is located. The sensor in question is preferably a surface sensor. It preferably possesses a plurality of picture elements which are preferably in a regular order. Preferably, the picture elements are ordered by lines along parallel lines and possess the same distance to each other. A CCD sensor is particularly suitable.
In an embodiment of EP 0 419 936 B1, a reference radiation generated according to the shearing method is superimposed on the sensor. This reference radiation is generated by a shearing optical system, for example an optical wedge or prism. The optical wedge or the prism is positioned prior to the lens in an embodiment of EP 0 419 936 B1. The optical wedge or the prism masks a part, preferably one half, of the lens or the aperture of the imaging optical system. The phase of the radiation from the body, that is the phase of the radiation which was reflected by or which passed through the body, is determined from the measurement signals or the intensity signals of the sensor or sensor elements (pixels).
The apparatus known from EP 0 419 936 B1 can also be called an electronic speckle pattern interferometer (ESPI). To allow a complete phase-angle measurement with one single shot, in EP 0 419 936 B1 the imaging optical system is designed or adjusted in such a way that the image of a speckle generated by the radiation on the body in the image plane covers at least three sensor elements.
From DE 195 13 233, a method and an apparatus are known for the determination of phases and phase differences of radiation, in particular of light radiation. In this method, an object is exposed in at least two states to coherent or partially coherent radiation of a pre-determined frequency. In each state, the radiation reflected or passed through is imaged by an imaging optical system in an image plane in which a sensor with a plurality of preferably regularly ordered sensor elements or pixels is located. A reference radiation having a defined, preferably identical frequency with a defined phase position is superimposed on the sensor. The phase difference of the radiation from or through the object between the two states is determined from the intensity signals of the sensor elements or pixels. The object beam and the reference beam are adjusted in such a way here that they generate an interference pattern with a preferably constant spatial carrier frequency. The method and apparatus in accordance with DE 195 13 233 A1 have the object of further developing the method and the apparatus known from EP 0 419 936 B1 in such a way that with one image shot per object state a high image resolution is achieved. In accordance with DE 195 13 233 A1, this object is to be solved by the imaging optical system being designed and adjusted in such a way that when speckles occur, the image of a speckle in the image plane generated by the radiation only covers around two sensor elements or pixels. The corresponding intensity values recorded for each state from in each case only around two sensor elements or pixels are taken into account alternately or cross-ways for the determination of the phase difference. In addition to the phase difference, it is also possible to compute from at least two of these shots the phase, the contrast and the background brightness. In accordance with DE 195 13 233 A1, it is possible to generate the reference radiation from the object radiation in accordance with the shearing method. Although the performance of the method in accordance with DE 195 13 233 A1 is generally possible, the results that can be achieved with it have, however, substantially more noise. The signal/noise ratio is worse than with the method in accordance with EP 0 419 936 B1.
It is the object of the invention to improve a method and an apparatus of the type first given.
This object is solved in accordance with the invention by the imaging optical system possessing a diaphragm having an aperture, preferably a slit, or a diaphragm having two apertures, preferably two slits. By means of the aperture(s) of the diaphragm and the shearing optical system, it is possible to generate a spatial carrier frequency.
Advantageous aspects are described hereinbelow.
The diaphragm can possess a slit, preferably a rectangular slit. It is advantageous if the breadth of the slit can be adjusted. It is preferably a rectangular modulation diaphragm.
The diaphragm can, however, also be designed as a double slit. It then possesses preferably two rectangular slits. Preferably, the slits, which are disposed at a distance a to each other, each possess the same breadth b. It is furthermore advantageous if the distance a and/or the width b of the slits is/are able to be adjusted.
The shearing optical system which serves to generate the reference radiation in accordance with the shearing method, can possess a wedge or folding wedges or two tilted parallel plates. However, other optical elements are also possible to generate the shearing effect.
The wedge or folding wedges or the tilted parallel plates or the other optical element for the generation of the shearing effect can be disposed between the diaphragm and one or more or all lens elements or other optical elements of the imaging optical system. Instead of this or additionally, the diaphragm can be disposed between one or more or all lens elements or other optical elements of the imaging optical system and the wedge or folding wedges or the tilted parallel plates or the other optical element for the generation of a shearing effect.
A particularly advantageous aspect is characterised by the imaging optical elements being or having been adjusted in such a way that the distance a of the slit is of the same size as the breadth b of the slit. This advantageous aspect cannot only be used in embodiments having two slits (xe2x80x9cdouble-aperture), but also in embodiments where only one slit is present. In the latter case, namely, it is achieved by the imaging optical system that the sensor xe2x80x9cseesxe2x80x9d two slits.
A spatial carrier frequency which is generated by the interference of a double slit or a double-aperture diaphragm (see FIG. 2) or a Fresnel biprism (see FIG. 3) does not possess a constant period and has a poor contrast as the interference and diffraction effects always occur simultaneously and interfere with each other. The interference distribution of a double slit with Fraunhofer diffraction is described in the equations (1) to (3) given below:
I (xcex8)=4 I 0[(sin2xcex2/(xcex22)] cos2 xcex1xe2x80x83xe2x80x831)
xcex1=(ka/2) sin xcex8xe2x80x83xe2x80x832)
xcex2=(kb/2) sin xcex8xe2x80x83xe2x80x833)
The object covered by these equations and the values occurring in them are shown in FIG. 4. In the equations:
I=intensity
xcex8=angle
k=ordinal number of the first, second, third, . . . interference minimum
a=slit distance
b=slit breadth.
In accordance with the mentioned advantageous further aspect
a=b.
It follows from this that:
xcex1=xcex2
so that equation (1) becomes the following equation (1xe2x80x2):
I (xcex8)=4 I 0[(sin2(2xcex1))/(2xcex1)2]xe2x80x83xe2x80x831xe2x80x2)
It can be seen from this that the carrier frequency generated by interference and diffraction and not constant at axe2x89xa0b becomes through a=b a homogeneously distributed spatial frequency with a constant period. It can be seen from the term 2xcex1 that the frequency is doubled by this. Through a=b a substantial improvement can be achieved which is to be found in the fact that a constant carrier frequency with a much better contrast can be generated at the sensor or CCD chip. This carrier frequency can be generated in the lines (horizontal) or in the columns (vertical) of the sensor or CCD chip. However, it is also possible to generate them in an intermediate angle, which can be achieved by disposing the sensor or CCD chip tilted at an angle.
Another advantageous further aspect is characterised in that the period of the carrier frequency covers at least two picture elements of the sensor. It is therefore possible to work in accordance with the methods of DE 195 13 233 A1 or with the apparatuses described there.
It is advantageous if the period of the carrier frequency covers at least three picture elements of the sensor, that is if work is done in accordance with the methods of EP 0 419 936 B1 or with the apparatuses described there. In accordance therewith, a complete phase-angle measurement is possible with one single shot.
In accordance with another advantageous aspect, the period of the carrier frequency covers at least four picture elements of the sensor. In this way, the computation algorithms are made substantially simpler.
If the period of the carrier frequency covers at least five picture elements of the sensor, a further increase in precision is achieved.
The period of the carrier frequency in one line or column can be adjusted very precisely by the aperture size of the modulation diaphragm independently of the shearing optical system or the shear width. The shearing optical system or the shear width are adjusted so that the preferred minimum number of the picture elements of the sensor is covered.